There already exist numerous brake apparatuses fitted with a first hydraulic circuit powered by an associated hydraulic source and used for braking under normal conditions, and with an emergency hydraulic circuit powered by a different source of hydraulic pressure and used only in the event of a breakdown.
Traditionally, each of the two hydraulic circuits (normal and emergency) is connected to each of the various brakes via a brake valve which is a servo-valve that is electrically controlled by a cockpit unit which measures information corresponding to brake pedal depression, and in general also to the speed of the wheel in question. For the state of the art, reference can be made to the following documents: EP-A-0 443 213, U.S. Pat. Nos. 4,834,465, 5,050,940, 5,024,491, 3,926,479, FR-A-2 038 001, and DE-B-1 118 020. Another example of a particularly elaborate electro-hydraulic brake apparatus for a set of aircraft wheels is described in the Applicant's document U.S. Pat. No. 5,397,173.
The various brake apparatuses described in the above-mentioned documents thus comprise electrically controlled servo-valves of structure that always has two hydraulic stages interconnected by a hydraulic link and respectively associated with control and with distribution, the control stage acting on the slider of the distribution stage. The hydraulic control stage is electrically controlled, and the feed pressure input to the servo-valve is modulated in the distribution stage by through sections (nozzles) operating on the principle of hydraulic leakage that is generally at a rate of about one liter per minute. Such hydraulic leakage is always required in that kind of design, and the magnitude thereof makes it difficult to use such servo-valves when the hydraulic source is constituted by a hydraulic accumulator, as may be the case for an emergency circuit. The volume of fluid consumed by the permanent leakage of such servo-valves is no longer available for powering the brakes, thereby considerably diminishing the duration and number of times the brakes can be applied before the accumulator has been emptied.
Although the structure of presently-used braking servo-valves is generally well understood, specialists are fully aware of the drawbacks and operating constraints on brake valves of those types.
The operating pressure recovered at the outlet from a servo-valve, which pressure is a function of the magnitude of the reference current, is, in fact, obtained with relatively poor accuracy. Firstly, the mechanical adjustment of the servo-valve nozzles cannot achieve dispersion on utilization pressure of less than a few bars, or indeed of about ten bars at some points, given the non-linearity and hysteresis phenomena specific to any type of hydraulic equipment. Furthermore, the servo-valve is sensitive to fluctuations in the feed pressure and in the temperature of the hydraulic fluid, which can give rise to operating pressure variations of several bars, even in normal operation. Finally, mechanical wear of the nozzles inevitably gives rise to drift over time.